1. Field of the Invention
The present invention relates to a transmitter circuit in a communications device such as a mobile telephone or a wireless LAN device and an electronic device such as an audio device or a video device, and to a data converter and a data conversion method for use therein. More particularly, the present invention relates to a transmitter circuit, a communications device and an electronic device in which quantization noise can be suppressed, and to a data converter and a data conversion method for use therein.
2. Description of the Background Art
FIG. 28 is a block diagram showing a configuration of a conventional communications device 900. Referring to FIG. 28, the conventional communications device 900 includes a transmitter circuit 901, a receiver circuit 902, an antenna duplexer 903 and an antenna 904. A high-frequency signal to be transmitted is produced at the transmitter circuit 901, and is radiated into the air from the antenna 904 via the antenna duplexer 903. A high-frequency signal received by the antenna 904 is passed to the receiver circuit 902 via the antenna duplexer 903, and the received signal is processed. The antenna duplexer 903 may be a duplexer using, for example, a switch, a dielectric, a SAW (Surface Acoustic Wave) filter, an FBAR (Film Bulk Acoustic Resonator) filter, etc.
FIG. 29 is a block diagram showing an exemplary configuration of the transmitter circuit 901 in the conventional communications device 900 shown in FIG. 28. FIG. 29 schematically shows signal waveforms at different positions in the transmitter circuit 901. The transmitter circuit 901 is a type of a transmitter circuit that produces a signal to be transmitted through polar modulation. Referring to FIG. 29, the conventional transmitter circuit 901 includes a data production section 910, an angle modulator 920 and an amplitude modulator 930.
The data production section 910 converts a baseband signal of a rectangular coordinate system represented by I data (in-phase data) and Q data (quadrature-phase data) orthogonal to each other to a signal of a polar coordinate system, and outputs amplitude data and phase data. The amplitude data is inputted to the amplitude modulator 930. The phase data is inputted to the angle modulator 920.
The angle modulator 920 angle-modulates the received phase data to output a carrier wave. The carrier wave is inputted to the amplitude modulator 930.
The amplitude modulator 930 amplitude-modulates the carrier wave from the angle modulator 920 with the amplitude data from the data production section 910 to output the modulated signal. Thus, a signal to be transmitted is obtained. This type of modulation is called “polar modulation”.
FIG. 30 is a block diagram showing a configuration of the amplitude modulator 930. Referring to FIG. 30, the amplitude modulator 930 includes matching circuits 931 and 936, a transistor 932, a DC power supply 933 and bias circuits 934 and 935.
The angle-modulated wave from the angle modulator 920 is received via the matching circuit 931, amplified through the transistor 932, and outputted via the matching circuit 936. The matching circuits 931 and 936 are circuits for the matching between the input and the output of the transistor 932. The bias circuits 934 and 935 are circuits for supplying bias voltages to the base or gate and the collector or drain, respectively, of the transistor 932. A DC voltage is supplied from the DC power supply 933 to the base terminal of the transistor 932 via the bias circuit 934. The gain of the transistor 932 varies depending on the voltage supplied from the data production section 910. Thus, the amplitude modulation is realized by supplying to the transistor 932 a voltage in proportion to the amplitude data from the data production section 910.
However, with the transmitter circuit 901 shown in FIG. 29, the output signal may be distorted when, for example, the input power to the transistor 932 becomes high or low. FIG. 31 is an exemplary schematic diagram illustrating the cause of the distortion.
In FIG. 31, the horizontal axis represents the level of the input power to the transistor 932, i.e., the level of the amplitude data. The left vertical axis represents the level of the output power from the transistor 932. The right vertical axis represents the phase of the signal passing through the transistor 932 (hereinafter referred to as the “passing phase”).
In FIG. 31 showing the output power and the passing phase together in a single graph, elliptical markings with arrow heads are used to indicate which curve belongs to which vertical axis.
In an ideal transistor, the input power and the output power are proportional to each other. Moreover, in an ideal transistor, the passing phase is kept constant as the input power increases. In FIG. 31, such characteristics of an ideal transistor are represented by dotted lines on the high input power side. Thus, with an ideal transistor, the output power and the passing phase vary linearly across the entire input power range.
However, transistors in practice may not have linear characteristics across the entire input power range. As shown in FIG. 31, for input power levels greater than P, the output power and the input power are not proportional to each other and the passing phase does not stay constant. In other words, for input power levels greater than P, the transistor does not have linear characteristics.
If amplitude data is inputted in such a non-linear region, the output power is amplified non-proportionally and the phase is shifted. Thus, if amplitude data is inputted in the non-linear region, the output signal is distorted. In other words, as the output of the transmitter circuit increases, the output signal will be distorted.
In order to solve the problem, conventional methods discretize the amplitude data. FIG. 32 is a block diagram showing an alternative configuration of a transmitter circuit in the conventional communications device 900 of FIG. 28 (see FIG. 1 of Japanese Laid-Open Patent Publication No. 2002-325109). Referring to FIG. 32, a transmitter circuit 901a includes the data production section 910, the angle modulator 920, the amplitude modulator 930, a band-pass filter 940 and a delta-sigma modulator 950. In FIG. 32, like components to those shown in FIG. 29 are denoted by like reference numerals. FIG. 32 schematically shows signal waveforms at different positions.
In the transmitter circuit 901a, the amplitude data from the data production section 910 is delta-sigma-modulated through the delta-sigma modulator 950 so as to be discretized into a binary signal (typically using two values of zero and a positive real number), which is inputted to the amplitude modulator 930.
The phase data from the data production section 910 is inputted to the angle modulator 920, where the phase data is angle-modulated into an angle-modulated wave, which is inputted to the amplitude modulator 930.
The amplitude modulator 930 amplitude-modulates the carrier wave from the angle modulator 920 with the output signal from the delta-sigma modulator 950. The configuration of the amplitude modulator 930 is as shown in FIG. 30. Therefore, a voltage signal corresponding to the binary signal from the delta-sigma modulator 950 is supplied to the transistor 932, whereby the carrier wave is turned ON/OFF by the binary signal, thus realizing amplitude modulation.
The band-pass filter 940 outputs the signal to be transmitted while removing quantization noise introduced by the delta-sigma modulation.
As described above, the transmitter circuit 901a performs amplitude modulation using a delta-sigma-modulated binary signal. Therefore, the output signal of the amplitude modulator 930 is obtained simply by turning ON/OFF an angle-modulated wave. Thus, the output signal from the transmitter circuit 901a will not be distorted.
However, the transmitter circuit 901a shown in FIG. 32 has a problem in that the output signal contains considerable quantization noise. FIG. 33 shows the spectrum of an output signal from the amplitude modulator 930 of the transmitter circuit 901a shown in FIG. 32. FIG. 34 shows the spectrum of an output signal from the band-pass filter 940 of the transmitter circuit 901a shown in FIG. 32. Note that the zero frequency point along the horizontal axis represents the center frequency.
As shown in FIG. 33, the output from the amplitude modulator 930 contains considerable quantization noise. Therefore, the quantization noise needs to be removed through the band-pass filter 940. However, removing the quantization noise results in a loss of signal. Where the quantization noise energy accounts for 30% to 40% of the total energy, for example, the overall efficiency of the transmitter circuit 901a will be as low as 60% to 70% even if the efficiency of the amplitude modulator 930 is 100%. Therefore, the overall power consumption of the transmitter circuit cannot be reduced unless the power of the quantization noise is reduced. In other words, the higher the power of the quantization noise is, the higher the overall power consumption of the transmitter circuit is.
Moreover, a large amount of suppression is required at the band-pass filter 940 in order to sufficiently remove the unnecessary quantization noise. To do so, the filtering loss through the band-pass filter 940 increases. In order to realize a large amount of suppression, the size of the band-pass filter 940 increases, thus increasing the overall circuit scale of the transmitter circuit.
Moreover, as shown in FIG. 34, quantization noise near the intended wave frequency cannot be removed by a band-pass filter. In order to remove the quantization noise near the intended wave frequency, the delta-sigma modulator 950 needs to output a signal that is sufficiently low in noise. In order to realize this, the level of the quantization noise near the intended wave frequency needs to be reduced by increasing the clock frequency of the delta-sigma modulator 950. This however increases the power consumption at the delta-sigma modulator 950.